1. Field of the Invention
This invention generally relates to an auto-focusing system, for example, for use in a photographic camera, and, in particular, to an auto-focus arithmetic device capable of carrying out a focus aid operation in the manual focusing mode.
2. Description of the Prior Art
Use is typically made of an auto-focus arithmetic device in an auto-focus camera and a camera provided with a manual focus aid display. In the conventional auto focus arithmetic device, the arithmetic operation to determine the amount of discrepancy from the in-focus condition is typically carried out as divide between an integer portion and a fractional portion. In this case, the arithmetic operations for the integer and fractional portions are carried out in parallel to thereby move a photographic lens automatically or manually until the lens is brought into the in-focus condition whereby the in-focus condition is indicated in the display. Thus, in the prior art auto-focus arithmetic device, even if the amount of discrepancy to the in-focus position is large, the fractional portion as well as the integer portion is being calculated, which tends to slow down the focus aid display operation during the manual focusing mode.